Understanding Bike Geometry Measurements

A guide to the terminology used throughout GeometryGeeks

Top Tube Length
aka: Effective Top Tube, Horizontal Top Tube, Virtual Top Tube

Top tube is a good indicator of overall size of the bike.

It's measured 'effectively' horizontally from head tube axis to seat tube axis.

Older bikes frequently had horizontal top tubes, but now sloping tubes are much more common. Where the top tube is horizontal the Effective Top Tube and Actual Top Tube will be identical.

A longer top tube has you more stretched out on the bike, all other things being equal. This will give you a racier, more aerodynamic position on a road bike - possibly at the expense of all-day comfort.

In mountain biking, top tubes have been lengthening as stems get shorter and bars wider, giving a more stable ride at speed.

Actual Top Tube Length

Top tube length as measured from head tube axis to seat tube axis, along the tube itself.

This is not particularly useful for bike fit.

Seat Tube Length

"Centre to Top" or C-T is the length from bottom bracket centre to top of seat tube. Useful because you can measure it and helps you work out how much seatpost you need.

"Centre to Centre" or C-C is the length from bottom bracket centre to the middle of where the top tube meets the seat tube. Was a bit more useful when bikes were made without sloping top tubes.

"Effective" is the length from the bottom bracket centre to the point where a horizontal line from the top of the head tube meets the seat tube axis.

One of these is often used as an overall measure of a bike's size.

Head Angle
aka: Steerer Angle

The angle the forks make with the ground.

A slacker (smaller) head angle gives more stable handling at speed but can feel wander-y while going slower. A steeper (larger) angle makes a bike more precise at slow speeds, but twitchier when going faster.

Seat Angle
aka: Seat Tube Angle, Effective Seat Tube Angle

Seat tube angle. Steeper (larger) places you further forward while seated, slacker (smaller) stretches you out more.

On mountain bikes where seat tube does not intersect the BB, this is the 'effective' angle.

Actual Seat Tube Angle

On mountain bikes where seat tube does not intersect the BB, this is the actual angle.

Reach

The horizontal distance from the centre of the bottom bracket to the centre-top of the head tube, where the virtual steering axis passes through.

Very useful for gauging how long a bike feels.

Goes hand-in-hand with...

Stack

The vertical distance from the centre of the bottom bracket to the centre-top of the head tube, where the virtual steering axis passes through.

A low stack feels low, aero and racy. Go too low and your back and neck might start to ache on longer rides, though.

Head Tube

Length of head tube.

A tall head tube may mean you can't fit forks that have had their steerer tube cut, such as those from a smaller bike. Make sure the fork steerer is long enough to allow for headset cups, stem height and head tube length.

A taller head tube raises your hand position, but Stack is often a better measure for this."

Chainstay
aka: Rear Centre

Distance from rear wheel axle to bottom bracket centre.

Front Centre

Distance from centre of bottom bracket to the front axle.

Wheelbase

Distance between front and rear axles.

Standover
aka: Standover Clearance, Standover Height

Standover height or clearance is the height from the ground to the top tube. Normally measured at a point halfway along the top tube.

BB Drop

How far the bottom bracket is below a line drawn between front and rear axles. Sometimes given as a negative number. Almost every bike ever will have a BB below that line.

A bigger drop lowers your centre of gravity, good for cornering but less good for catching your pedals!

BB Height

How far the bottom bracket is above the ground.

Describes the same thing as BB drop, but varies depending on tyre choice, suspension sag and other factors.

Fork Rake/Offset

How far in front of the steerer tube axis the front axle is.

Trail

The distance between the point where the projected steerer tube axis meets the ground and the point where the the wheel contacts the ground.

More trail feels more stable at speed, but can cause more 'flop' of the front wheel when turning.

Seatpost Size (Diameter)

Diameter of seatpost required for the frame. Some frames may require specific shapes of seatpost (e.g. aerodynamic profile) or use a fixed seatmast.

Seatpost Length

Length originally specified as part of the complete bike, or a dropper post.

Handlebar Width

Width originally specified as part of the complete bike.

Stem Length

Length originally specified as part of the complete bike.

Crank Length

Length originally specified as part of the complete bike.

BB Type
aka: Bottom Bracket Type

Bottom bracket standard, e.g. Pressfit BB30 or Threaded 73mm shell.

Axle Spacing

Rear axle spacing standard, e.g. 12x148 Boost or 135x9 QR.

Fork Length
aka: Axle to Crown, A2C

The distance from front axle to top of fork crown. May also be referred to as fork length, normally on road bikes (but not technically the same thing).

Wheel Size

Diameter of rim used, and tyre width if appropriate. e.g:

  • 700c road wheel
  • 26"/27.5" (650b)/29" MTB wheels
  • 29+ semi fat.

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